Differential amplifier opamps common-mode gain

[paulmdrdo]

I'm having a hard time deriving for equations 10-8 -10-9.

i tried adding equations 10-6 and 10-7 but still can't proceed to proper derivation. Any help is appreciated.

 

[BradtheRad]

i tried adding equations 10-6 and 10-7 but still can't proceed to proper derivation.

Sometimes I put a potentiometer at the inverting input, to make it easy to vary gain.

It also helps clarify something about how gain works in the op amp: The input grabs a portion of the incoming voltage (R1) and a portion of the output voltage (R2). Somewhere in the inverse feedback relationship (somewhere) is a certain amount of gain.

That function can be defined by a simple formula (-R2/R1), but then you need to throw in the effect of the non-inverting input, making it more complicated, doesn't it?

I suppose it's possible to merge the equations into one equation for some situations but maybe not for all situations.

I guess my reply doesn't solve your question but we can see that a lot goes on within an op amp.

 

[danadakk]

This might help, attached.

Regards, Dana.

 

[paulmdrdo]

This might help, attached.

Regards, Dana.

Hello! May I know what site did you get this design notes? thank you!

 

[danadakk]

That ap note I think came from the old Linear technology website. They are now
a part of analog devices.


Here is a great ref work handbook on IA amps. AD site has many many ap notes.

https://www.analog.com/media/en/training-seminars/design-handbooks/designers-guide-instrument-amps-complete.pdf

Attached a collection of stuff from Bob Pease, well known National Semi aps engineer. Passed a few years ago.

Bob Pease Lab Notes Part 9.pdf

PDF File

1drv.ms

Regards, Dana,